1. Field of the Invention
This invention relates to porous polytetrafluoroethylene (hereinafter "PTFE") materials having a unique and useful combination of high strength and coarse microstructure, and a method for producing these materials. Articles made from these materials are particularly suitable for use in the medical field.
2. Description of the Prior Art
The products of this invention derive from paste formed products of PTFE. Paste extrusion or paste forming techniques are old in the art and consist of mixing a coagulated dispersion of polytetrafluoroethylene resin with a liquid lubricant and forcing the mixture through an extrusion die or otherwise working the lubricated mixture to form a coherent shaped article. The lubricant is then removed, usually by drying, to form a porous, unsintered PTFE article having a density usually within the range of 1.4 to 1.7 gm/cc. Such densities correspond to porosities of 39% to 26%, respectively. At this stage, the article can be raised above its crystalline melt point of about 345.degree. C. to sinter it, coalescing the porous material to form a non-porous sintered article.
Alternatively, the unsintered article can be made more porous and stronger by stretching according to techniques taught in U.S. Pat. No. 3,953,566. Subsequent to stretching, the stretched article can be held restrained and heat treated above the crystalline melt point. In this instance, the article remains porous and when cooled a strong porous article of PTFE is obtained. In the discussions which follow, the term "sintering" is used interchangeably with the process step of raising the unsintered article above its crystalline melting point. U.S. Pat. No. 3,953,566 provides a method of producing all kinds of microporous stretched PTFE, such as films, tubes, rods, and continuous filaments. The articles are covered by U.S. Pat. No. 4,187,390. The microstructure of these articles consists of nodes interconnected by fibrils.
The key element of the U.S. Pat. No. 3,953,566 process is rapid stretching of PTFE. Rapid stretching allows the unsintered article to be stretched much farther than had previously been possible while at the same time making the PTFE stronger. The rapid stretching also produces a microstructure which is very fine in scale having, for example, a very small effective pore size. U.S. Pat. No. 3,962,153 describes very highly stretched products, stretch amounts exceeding 50 times the original length. The products of both the U.S. Pat. Nos. 4,187,390 and 3,962,153 patents have relatively high matrix tensile strengths. (See discussion of "matrix tensile strengths" and relation to article tensile strength and density in U.S. Pat. No. 3,953,566 at col. 3, lines 28-43.)
To compute the marix tensile strength of a porous specimen, one divides the maximum force required to break the sample by the cross sectional area of the porous sample, and then multiplies this quantity by the ratio of the density of the PTFE polymer component divided by the density of the porous specimen. The density of PTFE which has never been raised above its crystalline melt point is 2.30 gm/cc while the density of PTFE which has been sintered or raised above its crystalline melt point may range from above 2.0 gm/cc to below 2.30 gm/cc. For purposes of calculating matrix tensile strength in examples which follow, we have used a density of the PTFE polymer of 2.20 gm/cc for products which have been raised above the crystalline melt point, and a density of 2.30 gm/cc for those which have not.
When the unsintered articles are stretched at slower rates, either limited stretching occurs because the material breaks, or weak materials are obtained. These weak materials have microstructures that are coarser than articles that are stretched equivalent amounts but at faster rates of stretch. The term, "coarse," is used to indicate that the nodes are larger, the fibrils are longer, and the effective pore size is larger. Such coarse microstructures would have further utility if they were strong instead of weak.